Preparation And Supercapacitance Performances Of MXene-based Composites

Attributed to the ultra-high conductivity,superior hydrophilicity,abundant surface functional groups and adjustable layer spacing,MXene shows a broad application prospect in energy storage fields.However,due to the interaction of van der Waals forces between adjacent nanosheets and the existence of hydrogen bonds,MXene often suffers from irreversible face-to-face self-stacking and agglomeration,which leads to the reduced exposed active surface area and electrochemical performances.In order to solve the above problems,in this thesis,five kinds of MXene-based composites were prepared through different methods.The microstructure,morphology and surface properties of MXene-based composites were characterized,and the electrochemical properties of the electrode were also studied in detail.The main contents of the thesis are as follows:MXene/carbon black/carbon quantum dot（MXene/CB/C-Dot）composite was fabricated through electrostatic self-assembly approach using carbon black and carbon quantum dots as the raw material.The carbon black and carbon quantum dots bind tightly to MXene through electrostatic self-assembly.The presence of carbon black and carbon quantum dots inhibits the self-stacking of MXene nanosheets.In addition,this unique structure is conducive to the infiltration and transport of electrolyte ions,resulting improved electrochemical performance of the electrode.The effect of amounts of carbon black and carbon quantum dots on the microstructure and electrochemical performances of the material was investigated in detail.Electrochemical measurement results demonstrate that the composite with loading amounts of5%for carbon black and carbon quantum dots has the highest specific capacitance.The specific capacitance of MXene/CB/C-Dot composite electrode is 459 F g^?1 at 2 m V s^?1 with an impressive rate capability of 40.8%at 3 V s^-1.An asymmetric supercapacitor using N-doped holey graphene（NHRGO）as the positive electrode and MXene/CB/C-Dots as the negative electrode has been assembled.It can achieve an energy density of 36.9 Wh kg^?1 at the power density of 147.6 W kg^?1.The 3D macroporous oxidation-resistant Ti₃C₂T_x MXene/graphene/carbon nanotube（MXene/RGO/CNT,MRC）hybrid hydrogels were prepared through a mild gelation method with the assistance of L-cysteine as crosslinker and VC as reductant.The effect of amounts of graphene and carbon nanotube on the electrochemical performance of the MRC composites was investigated in detail.The MRC-30 aerogel exhibits remarkable specific capacitance of349 F g^-1 at 2 m V s^-1 and an impressive rate capability of 52.0%at 3 V s^-1.Notably,as-prepared3D macroporous MRC-30 aerogel also exhibits superior cycling stability with 97.1%capacitance retained after 100000 cycles at 200 m V s^-1.An asymmetric supercapacitor using N-doped graphene（NRGO）as the positive electrode and MRC-30 as the negative electrode has been assembled,which shows a specific capacitance of 81 F g^?1 at 2 m V s^-1.The presence of VC prevents the oxidation of MXene and the construction of 3D hydrogel structure is conducive to the diffusion and transport of ions in the electrolyte.The 3D porous oxidation-resistant MXene/graphene（PMG）architecture was prepared through a self-assembly approach with the assistance of metallic zinc powder as an in situ sacrificial template at room temperature.In the reaction process,the generated Zn²⁺in MXene and graphene mixed dispersion can break the electrostatic repulsion between MXene layers,which can serve as crosslinking sites to induce self-assembling between MXene and GO.Benefitting from the optimized structure,the oxidation of MXene is expected to be restricted,which greatly promotes rapid diffusion of electrolyte ions.The effect of amounts of graphene on the electrochemical performance of the PMG composites was investigated in detail.Electrochemical measurement results demonstrate that the composite with graphene loading of5%has the highest specific capacitance.The specific capacitance of PMG-5 composite electrode is 393 F g^?1 at a scan rate of 2 m V s^?1 with impressive rate capability of 32.7%at 10V s^-1.The PMG-5 electrode exhibits excellent cycling stability with almost no capacitance deterioration after 30000 cycles at a scan rate of 200 m V s^?1.An asymmetric supercapacitor has been assembled using NHRGO positive electrode and PMG-5 negative electrode,which shows an energy density of 50.8 Wh kg^?1.The V₂O₅ was synthesized by hydrothermal reaction using ammonium metavanadate as raw material,and the MXene/V₂O₅ composite was prepared by compounding with MXene through electrostatic self-assembly.MXene can inhibit the agglomeration of V₂O₅ and improve the electrical conductivity of composite.Electrochemical measurement results demonstrate that the composite with MXene loading of 5%has the highest specific capacitance(356 C g^?1 at 1A g^?1),which is significantly higher than those of V₂O₅(165 C g^?1)and the MXene(75 C g^?1).An asymmetric supercapacitor using Mn O₂/RGO as the positive electrode and MXene/V₂O₅-5as the negative electrode has been assembled,which shows a specific capacitance of 140 C g^?1at 0.5 A g^?1.It can achieve an energy density of 44.1 Wh kg^?1 at the power density of 176 W kg^?1.The MXene/Bi₂O₃ composite was synthesized by the combined solvothermal and electrostatic self-assembly method.The presence of MXene inhibits the agglomeration of Bi₂O₃nanoparticles,which is conducive to the infiltration and transport of electrolyte ions,and shorten the transport path of electrolyte ions and electrons.The effect of MXene content on the electrochemical performances of all samples were investigated in detail.Electrochemical measurements demonstrate that the composite with MXene loading of 1%has the highest specific capacitance.The MXene/Bi₂O₃-1 negative electrode material can display a high specific capacitance of 1359 C g^?1 at 1 A g^?1,while a capacitance retention of 70%can be obtained even at 10 A g^?1.An asymmetric supercapacitor using Ni Co layered double hydroxide/graphene（Ni Co-LDH/RGO）as the positive electrode and MXene/Bi₂O₃-1 as the negative electrode has been assembled.With the voltage range of 1.6 V,the energy density of as-assembled device can reach up to 65.9 Wh kg^?1.